Automatic transmissions, especially for motor vehicles, include planetary gear sets according to the state of the art, which are shifted using friction or shifting elements, such as clutches and brakes, and are usually connected with a starting element subject to a slipping action and are optionally provided with a bridging clutch, such as a hydrodynamic torque converter or a hydraulic clutch.
A transmission of this type emerges from EP 0 434 525 A1. It basically includes a drive shaft and an output shaft, which are arranged parallel to each other, and a double planetary gear set arranged concentrically in relation to the output shaft, and five shifting elements in the form of three clutches and two brakes, whose selective blockage respectively determines in pairs the various gear ratios between the drive shaft and the output shaft. Hereby, the transmission has a control gear and two power paths, so that six forward gears can be attained through the selective engagement in pairs of the five shifting elements.
Hereby two clutches are needed in connection with the first power path to transmit the torque from the control gear set to two elements of the double planetary gear set. These are basically arranged behind the control gear set in the direction of the double planetary gears in the direction of the flow of force. A further clutch is provided in connection with the second power path, which detachably connects this with a further element of the double planetary gears. Hereby the clutches are arranged in such a way that the internal disk support forms the output.
Furthermore, a compact multi-step reduction gear in planetary construction, especially for a motor vehicle, is known from the publication U.S. Pat. No. 6,139,463, which has two planetary gears and a control gear set, as well as three clutches and two brakes. Two clutches C-1 and C-3 are provided with this known multi-step reduction gear in connection with a first power path for transmitting the torque from the control gears to the two planetary gears. Here the external disk support or the cylinder or the piston or the pressure compensation side of clutch C-3 is connected with a first brake B-1. Moreover the internal disk support of the third clutch C-3 is connected with the cylinder or piston or pressure compensation side of the first clutch C-1, whereby the internal disk support of the first clutch C-1 is arranged on the output side and is connected with a sun wheel of the third planetary gear set.
Moreover, a multi-step reduction gear is known from DE 199 49 507 A1 of the Applicant, in which two non-shiftable control gears are provided on the drive shaft, which generate two RPMs on the output side that can, in addition to the RPM of the drive shaft, be shifted electively to a shiftable double planetary gears acting on the output shaft through selective closing of the shifting elements used in such a way that in each case only one shifting element of the two shifting elements just activated must be engaged or disengaged for shifting from one gear into the respectively next following higher or lower gear.
Furthermore, an automatically shiftable motor vehicle transmission with three single rod planetary gears, as well as three brakes and two clutches for shifting from six forward gears and one reverse gear and, with a drive as well as an output shaft, is known from DE 199 12 480 A1. The automatically shiftable motor vehicle transmission is constructed in such way that the drive shaft is directly connected with the sun wheel of the second planetary gear and that the drive shaft can be connected with the sun wheel of the first planetary gears through the first clutch and/or through the second clutch with the rod of the first planetary gears. In addition or as an alternative, the sun wheel of the first planetary gears can be connected through the first brake with the housing of the transmission and/or the rod of the first planetary gears through the second brake with the housing and/or the sun wheel of the third planetary gears through the third brake with the housing.
The present invention is based upon the objective of proposing a multi-step reduction gear of the type stated at the beginning in which the construction expenditure is optimized and, moreover, the degree of efficiency in the main driving gears is improved with respect to drag and gearing losses. Additionally, low torques should be acting on the shifting elements and planetary gears in the multi-step reduction gear of the invention and the RPMs of the shafts, shifting elements and planetary gears should be kept as low as possible. Furthermore, the number of gears as well as the transmission ratio spread should be increased.